In the semiconductor industry, it is well known that germanium (Ge) has higher carrier mobility than silicon (Si) for both electrons and holes. Despite having higher carrier mobility, Ge substrates are not currently being used in the fabrication of metal oxide semiconductor field effect transistors (MOSFETs) due to the general poor quality of germanium oxide.
Advances in Si technology have lead to the introduction of high-k dielectrics (having a dielectric constant greater than SiO2) as the MOSFET gate insulator. The high-k dielectrics are also expected to be usable with Ge, thus removing the main obstacle in realizing a Ge-based FET. In addition to having high electron and hole mobility, germanium has other advantages such as lower contact resistance and lower dopant activation temperatures than those required by silicon, thus facilitating the formation of shallow junctions.
The higher device performance obtained with silicon-on-insulator (SOI) substrates can also be obtained with germanium-on-insulator (GOI) substrates. Additionally, since current fabrication labs are equipped with tools designed to handle Si substrates, it is desirable that GOI stacks be formed on a Si wafer.
Germanium can also be used to realize fast optical detectors for commonly used wavelengths such as 1.3 microns and 1.55 microns. A Ge photodiode implemented on a GOI substrate can be designed to have lower parasitics, and higher quantum efficiency at a given wavelength. In such a structure, it is possible to replace the insulator with an insulating Bragg mirror that can further increase the photodetector responsively. Since Si is transparent at these wavelengths, backside illumination of a Si wafer having a Ge diode is possible.
The poor quality of germanium oxide makes it difficult to bond Ge to SiO2 by direct bonding since the adhesion between Ge and SiO2 is poor. Another limiting factor for consideration with germanium oxides is that Ge has a relatively low melting temperature (approximately 937° C.), which forces one to use low bonding temperatures (on the order of about 650° C. or less). A still other problem with germanium oxides is that germanium oxides are soluble in water therefore during cleaning in an aqueous media germanium oxide can be removed.
One possible approach for fabricating GOI substrates is to use the SMARTCUT technique described by Colinge, J-P, Silicon-on-Insulator Technology, 2nd Ed., Kluwer Academic Publishers, 1997. In the SMARTCUT technique, a thin Ge layer is transferred from a Ge wafer (i.e., the donating wafer) onto a handle wafer. The Ge wafer typically includes a hydrogen implant region formed therein. The Ge wafer is bonded to a handle wafer and an annealing step is performed to strengthen the initial bond and to obtain blistering at the depth of the hydrogen implant. As a result, the Ge layer separates from the donating Ge wafer and remains bonded to the handle wafer. The donating Ge wafer is not lost after bonding and can be used many times for further bonding as the source of the GOI substrate material.
Despite the capability of using the SMARTCUT approach in fabricating GOI substrate materials, the above problems with germanium oxides is still prevalent. Hence, there is a need for providing a new and improved method for fabricating Ge-on-insulator substrate materials that avoids the formation of germanium oxides.